Conventionally, as a method for machining a pipe such as a steel pipe into a smaller diameter pipe, a cold drawing work has been practiced, in which a pipe is drawn through a die with a plug being inserted into the pipe. When drawing work of a pipe is performed with a drawing machine, a stick-slip phenomenon may occur during drawing due to the mechanism of such machining.
The stick-slip phenomenon will be described with reference to FIG. 1.
A plug 3, which is inserted into a pipe T, is provided at a front edge of a plug support bar 4, and a rear edge of the plug support bar 4 is fixed on a base of a drawing machine. During drawing, a carriage (not shown) attached to the front edge of the pipe T pulls the pipe T in the drawing direction. At this moment, the plug 3 is pulled by friction force generated between itself and the inner surface of the pipe T, thereby being moved in the drawing direction integrally with the pipe T. When the plug 3 is pulled and moved in the drawing direction, the plug support bar 4 stretches in the drawing direction since the rear edge of the plug support bar 4 is fixed on the base of the drawing machine. Accordingly, on account of a contractive force due to the elasticity of the plug support bar 4, the plug 3 is subject to a force to pull it back to the opposite side (the plug support bar 4 side) in the drawing direction. As the moved distance of plug 3 in the drawing direction increases, the contractive force due to the elasticity of the plug support bar 4 increases as well so that the force to pull back the plug 3 increases. When the force to pull back the plug 3 becomes larger than the friction force generated between the inner surface of the pipe T and the plug 3, slip occurs between the plug 3 and the inner surface of the pipe T so that the plug 3 is pulled back to the plug support bar 4 side. When the plug 3 is pulled back and thereby the contractive force of the plug support bar 4 decreases, the plug 3 is again pulled by the pipe T to be moved in the drawing direction. In this way, the movement of the plug 3 in the drawing direction and the pulling back thereof to the plug support bar 4 side are repeated so that the plug 3 vibrates along the drawing direction. The stick-slip phenomenon is a phenomenon in which the plug 3 significantly vibrates along the drawing direction due to the friction and slipping between the plug 3 and the pipe T during drawing, thereby generating a sound. This stick-slip phenomenon is likely to occur when the drawing speed is large or when the lubricity between the pipe and the plug is deficient.
Occurrence of such a stick-slip phenomenon will result in dimensional defects in which the outer diameter and inner diameter dimensions of the pipe after drawing vary in the longitudinal direction of the pipe. When the stick-slip phenomenon is significant, not only dimensional defects but also crack flaws will occur.
Since the occurrence of a stick-slip phenomenon leads to the generation of sound attributable to vibration of the plug etc., the operator reduces the drawing speed upon hearing the sound of a stick-slip phenomenon during drawing. Thus, for subsequent pipes in the same lot, drawing is performed at a speed not more than the reduced drawing speed, thereby preventing the occurrence of a stick-slip phenomenon. However, as the result of being too much concerned about the occurrence of a stick-slip phenomenon, there is a risk that the drawing speed is reduced more than necessary, and if so, the manufacturing efficiency will be reduced.
Moreover, since the detection of a stick-slip phenomenon relies on the auditory sense of the operator, the accuracy of the detection is not sufficient. Furthermore, since there is difference in the detectability between operators, there is a risk that the reaction such as reducing the drawing speed may be delayed when a stick-slip phenomenon occurs. For this reason, conventionally, there have been proposed various methods for detecting such a stick-slip phenomenon as described above without relying on the auditory sense of the operator.
For example, there is proposed a drawing method in which an AE sensor is attached to a die and it is judged that a stick-slip phenomenon has occurred upon detection of a vibration of a predetermined frequency (refer to Patent Literature 1).
Moreover, there is proposed a detection method in which the strain of a carriage which pulls the pipe is measured, and the occurrence of a stick-slip phenomenon is judged from the result of frequency analysis of the amount of change in the strain (refer to Patent Literature 2).
As described so far, the occurrence of a stick-slip phenomenon can be detected at a tolerable level by a method in which the operator makes judgment based on sound as described above, as well as the methods of Patent Literatures 1 and 2. However, since the dimensional defects of the pipe have already occurred at the time when a stick-slip phenomenon occurs, it is desirable to detect a precursor of the stick-slip phenomenon (hereafter, a precursor of a stick-slip phenomenon is also abbreviated simply as a precursor) in a stage prior to the occurrence of a stick-slip phenomenon. Detecting a precursor and reducing the drawing speed before a stick-slip phenomenon occurs make it possible to effectively prevent the occurrence of a stick-slip phenomenon.